The present invention relates to a transformer for performing impedance transformation or for performing separation of a ground circuit in a microwave integrated circuit. The transformer includes an inductor element formed on a substrate which is used for blocking (shunting) or passing a high frequency signal or for constituting a filter in combination with a capacitance element and/or a resistor element, for processing a high-frequency signal of from several hundreds MHz to several tens GHz.
With rapid advances in the development of information network systems, satellite communication system in a high frequency band are becoming more popular. As a result, a high-frequency field-effect transistor, a Schottky barrier type field-effect transistor (MESFET) using a compound semiconductor such as a GaAs semiconductor or the like is being used more and more. Recently, in order to reduce the size and cost of the system for such communication and in order to improve system performance, a first-stage amplifier portion of a down converter for converting a high-frequency signal into a low-frequency signal has been developed and fabricated into an integrated circuit (MMIC: monolithic microwave integrated circuit). The MMIC provides a communication device replacing one constituted by a large number of separate elements. The use of such an MMIC reduces the number of parts, the mounting costs and improves reliability by reducing the number of connection points required by the circuit. Compared with prior devices using a large number of separate elements, reduction in cost can be easily achieved.
Although it is required that a transformer for performing impedance transformation or performing separation of a ground circuit be provided in the form of a MMIC, in a conventional MMIC, an active element has been used to perform the transformer function. The use of these active elements such as an inductor element using a distributed constant line element and a spiral inductor, however, create problems. For example, when using the distributed constant line element such as a micro strip line, there is a tendency for the area of the strip line to become large. This tendency becomes significant in a MMIC for use at low frequencies. As the size of the MMIC chip becomes large, production yield becomes low and the number of chips which can be obtained from a semiconductor substrate becomes relatively smaller, thus, increasing the cost per chip. In the case of a spiral inductor constituted by spirally shaping a line with a width of about 2 .mu.m-about 20 .mu.m, it becomes impossible to form a transformer in view of the inductor structure. Accordingly, a transformer cannot be effectively achieved by using an active element.
The pseudo transformer using an active element reduces the size of the MMIC but increases electric power consumption. Therefore, it is not always desirable to use an active element.